A 1 BAR MAP sensor reports barometric pressure times one, which is approximately 14.7 psi or 30in/Hg (inches of mercury). 2 BAR or 3 BAR simply mean two or three times above barometric pressure. The sensor will measure each direction, vacuum or pressure.

For Reference:
1 BAR is for a normally aspirated engine
2 BAR will handle a forced induction up to 14.5 psi of boost
3 BAR will handle a forced induction up to 29.0 psi of boost
4 BAR will handle a forced induction up to 43.5 psi of boost

 A vehicle's computer is calibrated to function at a specific level as programmed by the design engineer. A 2, 3, or 4 BAR MAP sensor placed in a system designed for a 1 BAR sensor will not function correctly; in fact the system will run very rich because of the low return voltage signal. A computer that is programmable is a must for any tuner attempting to boost their engine's output with the addition of a turbocharger and a 2, 3, or 4 BAR MAP sensor.